Eventually the government is going to come after him. If you just happen to be sitting on 400 million without paying taxes the government is going to be interested.
For instance, if I take a piece of paper and some paint, and use them to make a painting that I hang on my wall, and this painting would sell for $100k if I elected to sell it, I do not have to report $100k of income. I only have taxable income when I actually sell the painting.
He might have $400m worth of bitcoins, but can he actually sell them and get $400m for them? It would be tricky.
When he mined them and they were worthless? When he sells them?
I think he either owes couple of cents or nothing.
In this case, his basis is $0 (his cost to acquire the asset). Were he to sell all of them, his profit is essentially 100%; however, I'm not entirely sure how bitcoin gains are codified. If they're regular capital gains, then it's done around that cost basis ($0 in this case), but if they aren't, then I don't know.
There hasn't been much formal on "purchase price" of mined coins as far as I know/remember. It's probably safest to treat them as a purchase price of zero, thus if you sell mined coins for $100, then you have a $100 income that you owe tax on. A clever accountant might be able to claim that mining hardware, electricity, etc, is the cost of acquisition, and so offset the sale price in that way, but it probably isn't worth the trouble for Satoshi or anyone else mining in the early days.
However he doesn't owe anything as long as he just holds the coins.
Just as you don't owe any taxes on inventory of goods that you have produced from raw materials.